1. Field of the Invention
The present invention relates to an image distortion correcting device used in a CRT (Cathode-Ray Tube) display device.
2. Description of the Background Art
 less than Intermediate Pincushion Distortion of Vertical Lines greater than 
FIG. 18 is a schematic diagram showing pincushion distortion of vertical lines in the intermediate areas (hereinafter referred to as xe2x80x9cintermediate pincushion distortionxe2x80x9d), which is often studied as a problem in CRT display devices used in current high-definition display monitors etc. In this diagram, vertical lines 23 are shown near the edges of the screen 20 and vertical lines 24 are shown in the intermediate areas of the screen (the intermediate areas between the central area and the marginal areas in the screen). When the vertical lines 23 in the marginal areas of the screen look approximately straight, the vertical lines 24 in the intermediate areas of the screen look distorted in a pincushion-like shape, which is called intermediate pincushion distortion.
It is difficult to solve such intermediate pincushion distortion only by controlling the magnetic field distribution of the deflection yoke.
Methods for correcting the intermediate pincushion distortion using circuitry include the well-known method of improved S-correction where the amount of S-correction is varied in the vertical scanning period (hereinafter simply referred to as vertical period).
 less than S-correction greater than 
First, the S-correction is described. The S-correction is a correcting method in which the horizontal deflection current is modulated from a sawtooth form to an approximately S-shaped form to obtain appropriate linearity in the horizontal direction.
FIG. 19 is an explanation diagram showing display condition on the screen 20 with a horizontal deflection current IH having a sawtooth waveform. As shown in this diagram, when the sawtooth horizontal deflection current IH flows as shown in the graph G11, the amount of displacement, X, of the electron beam path 25 varies as shown in the graph G12 in the horizontal section of the display, e.g. a CRT.
At this time, the marginal areas of the screen 20 (the screen is rotated counterclockwise by 90xc2x0) are more distant from the deflection center than the intermediate areas, so that, if the variation of the amount of deflection current is constant, the electron beam is deflected larger in the marginal areas than in the intermediate areas. Accordingly, the interval xcex94B between the vertical lines (horizontal lines in the diagram) in the marginal areas of the screen is larger than the interval xcex94A between the vertical lines in the intermediate areas of the screen (i.e. xcex94A less than xcex94B).
FIG. 20 is an explanation diagram showing display condition on the screen 20 with a horizontal deflection current IH having an S-shape-corrected sawtooth waveform. In order to correct the horizontal linearity shown in FIG. 19, the sawtooth horizontal deflection current IH shown in the graph G11 of FIG. 19 is modulated into an approximately S-shaped form as shown in the graph G21 in FIG. 20.
When the horizontal deflection current IH is modulated into such approximately S-shaped form, the S-shaped current provides a larger amount of current than the sawtooth current in the intermediate areas and therefore the lack of deflection in the intermediate areas of the screen can be compensated for as shown in the graph G22 of FIG. 20. An at the ends of the Y axis. While the combined inductance Ls is determined by the difference between the amounts of variations of the two coils"" inductances L1 and L2 from those at the ends of the Y axis, it is usually equal to or smaller than those at the ends of the Y axis.
As shown in FIG. 10, the condition in the display position TR (in the upper right corner of the screen) or the display position BR (in the lower right corner of the screen) can be regarded as overlap of the condition at the ends of the Y axis and that in the display position R. In this case, the horizontal correction coil L1 becomes closer to the saturation state than at the ends of the Y axis, so that the inductance L1 becomes smaller than at the ends of the Y axis; the saturation state of the horizontal correction coil L2 is further canceled than at the ends of the Y axis so that the inductance L2 becomes larger than at the ends of the Y axis. The combined inductance is equal to or smaller than those at the ends of the Y axis.
FIG. 11 is an explanation diagram showing the variations of the combined inductance of the horizontal correction coils with respect to the display positions on the screen. In this diagram, the vertical axis shows the combined inductance Ls and the horizontal axis shows the horizontal deflection current IH. In the two curves, the curve LC1 shows the variation of the combined inductance Ls corresponding to a lateral line in the top and bottom of the screen and the curve LC2 shows the variation of the combined inductance Ls corresponding to a lateral line in the middle area of the screen. The signs showing the display positions on the screen attached to the points on the graph are the same as those shown in FIG. 4.
As can be seen from FIG. 11, in the top and bottom of the screen, the combined inductance Ls is the largest at the end of the Y axis and it becomes smaller as it approaches the corners of the screen. In the middle area of the screen, the combined inductance Ls is appropriate horizontal linearity (i.e. xcex94A=xcex94B) can thus be obtained as shown in the screen 20 of FIG. 20 by appropriately controlling the approximately S-shaped waveform.
FIG. 21 is a circuit diagram showing an example of the circuit configuration of a horizontal deflection circuit having the S-correcting function. As shown in this diagram, the positive side of the power supply E0 is connected to a fly-back transformer 41 and its negative side is connected to the emitter of a horizontal output transistor Q1. The collector of the horizontal output transistor Q1 is connected to the primary side of the fly-back transformer 41 and its base receives a control pulse.
A diode D20, a capacitor C20, and a series connection of a horizontal deflection coil LH and an S-correction capacitor CS are connected in parallel to the horizontal output transistor Q1. A diode D21 is connected to the secondary side of the fly-back transformer 41 and rectifies a signal transformed by the fly-back transformer 41.
In this configuration, in order to modulate the horizontal deflection current into approximately S-shaped form, a pulse having the horizontal scanning periods (hereinafter simply referred to as xe2x80x9chorizontal periodsxe2x80x9d) is supplied to the base of the horizontal output transistor Q1, and the horizontal deflection coil LH and the S-correction capacitor CS series-connected thereto are made to resonate at a resonant frequency determined by the horizontal deflection coil LH and the capacitor C20, and then a parabolic voltage as shown in FIG. 22 appears at both ends of the S-correction capacitor CS. The amount of current becomes larger in the period where the voltage is high, i.e. in the scanning period in the intermediate part in the horizontal direction on the screen, and the sawtooth current is thus modulated into the approximately S-shaped horizontal deflection current IH.
 less than Intermediate Pincushion Distortion Correction with Vertical Modulation of S-correction greater than 
For convenience of description, linearity where xcex94A less than xcex94B as shown in FIG. 19 is called outer-area expansion and linearity where xcex94A greater than xcex94B is called inner-area expansion.
Considering the intermediate pincushion distortion from the viewpoint of the horizontal linearity, it is seen that the inner-area expansion occurs in the top and bottom areas of the screen and the outer-area expansion occurs in the middle area of the screen. The inner-area expansion can be regarded as excess of S-correction and the outer-area expansion can be regarded as lack of S-correction. Hence, the intermediate pincushion distortion can be corrected by making the amount of S-correction smaller in the top and bottom areas of the screen and larger in the middle area. That is to say, it can be corrected if the amount of S-correction can be appropriately varied in the vertical period.
 less than Structure of Conventional Image Distortion Correcting Circuit with S-correction Vertical Modulation Function greater than 
FIG. 23 is a circuit diagram showing the structure of a conventional image distortion correcting circuit shown in Japanese Patent Application Laid-Open No. 11-261839 (1999), for example. As shown in this diagram, a horizontal deflection current IH flows between the terminal P1 and terminal P2, where a horizontal deflection coil 21, horizontal correction coil L13 and horizontal correction coil L14 are series-connected between the terminals P1 and P2. The horizontal correction coil L13 and the horizontal correction coil L14 are wound around the same core 13. The horizontal deflection coil 21 is shown as a block for convenience, since various internal configurations, such as a single coil, a parallel connection of coils, etc. are possible.
A vertical deflection current IV flows between the terminal P3 and the terminal P4, where a vertical deflection coil 22 is provided between the terminal P3 and intermediate terminal P11. The vertical deflection coil 22 is shown as a block for convenience, since various structures, such as a combinational circuit of a single coil, or a series connection of coils, and resistors for balance correction (including variable resistors), etc. are possible.
The intermediate point P11 is connected to the anode of a diode D7 and to one end of a resistor R4, and the cathode of the diode D7 is connected to one end of a vertical correction coil L15 and the cathode of a diode D8. The other end of the resistor R4 and one end of a resistor R5 are connected to the other end of the vertical correction coil L15 and the anode of the diode D8 and the other end of the resistor R5 are connected to the terminal P4. The vertical correction coil L15 is wound around the core 13.
The intermediate pincushion distortion correcting saturable reactor unit 10 includes the horizontal correction coils L13 and L14, vertical correction coil L15, magnets 11 and 12, and core 13; the magnets 11 and 12 are arranged at both ends of the core 13 so as to bias the magnetic field in one direction (to the left in FIG. 23). The winding directions of the horizontal correction coils L13 and L14 are set in opposite directions to each other so that they produce magnetic fields in opposite directions to each other, and the winding direction of the vertical correction coil L15 is set so that it produces a magnetic field in the direction opposite to the direction in which the magnets 11 and 12 bias.
In the intermediate pincushion distortion correcting saturable reactor unit 10 in this image distortion correcting circuit, the inductances of the horizontal correction coils L13 and L14 where the horizontal deflection current IH flows is controlled in accordance with the vertical deflection current IV flowing through the vertical correction coil L15, so that the amount of S-shape distortion correction in horizontal deflection varies according to the amount of vertical deflection.
That is to say, while the horizontal correction coils L13 and L14 are connected to one end of the horizontal deflection coil 21 and are modulated by the vertical deflection current IV which varies in the vertical period, the vertical correction coil L15 produces a magnetic field directed to cancel the bias magnetic field produced by the magnets 11 and 12, whereby the inductances of the horizontal correction coils L13 and L14 are varied to control the correction. At this time, the horizontal deflection current IH applied to the two horizontal correction coils L13 and L14 is an S-corrected sawtooth current given in each horizontal period, and the vertical deflection current IV applied to the vertical correction coil L15 is a current where a sawtooth current given in each vertical period is rectified to the same polarity through the two diodes D7 and D8 and the two resistors R4 and R5.
FIG. 24 is a schematic side view showing the main structure of the intermediate pincushion distortion correcting saturable reactor unit 10. As shown in this diagram, the three drum-like partial cores 13a to 13c accommodated in the case 39 are arranged adjacent to each other on the same axis; the horizontal correction coil L13 is wound around the partial core 13a, the vertical correction coil L15 is wound around the partial core 13b, and the horizontal correction coil L14 is wound around the partial core 13c. 
The pair of magnets 11 and 12 are provided at the ends of the partial cores 13a to 13c with their polarities directed in the same direction. As stated above, the horizontal correction coils L13 and L14 are wound in opposite directions and the vertical correction coil L15 is wound in such a direction that, when a current is passed, a magnetic field is produced to cancel the magnetic field produced by the pair of magnets 11 and 12 (hereinafter referred to as bias magnetic field).
 less than Functions of the Conventional Device greater than 
In the conventional image distortion correcting circuit constructed as shown in FIGS. 23 and 24, the magnetic field produced from the vertical correction coil L15 in the vertical period cancels the bias magnetic field from the magnets 11 and 12, so that the inductances of the horizontal correction coils L13 and L14 vary. That is to say, the bias magnetic field is canceled in the top and bottom areas of the screen and therefore the combined inductance of the horizontal correction coils L13 and L14 becomes larger; the bias magnetic field remains in the middle area of the screen and therefore the combined inductance of the horizontal correction coils L13 and L14 becomes smaller.
By the way, the voltage waveform at both ends of the S-correction capacitor CS in the horizontal period is considered to be part of the sine wave caused by series resonance of the horizontal deflection coil 21, horizontal correction coils L13 and L14 and S-correction capacitor CS, so that its resonant angular frequency xcfx89s is given as shown by the equation (1) below:
xcfx89s=1/{square root over ((Lh+Ls)xc2x7Cs)}xe2x80x83xe2x80x83(1)
Where Lh is the inductance of the horizontal deflection coil 21, Ls is the combined inductance of the horizontal correction coils L13 and L14, and Cs is the capacitance of the S-correction capacitor CS.
It is seen from equation (1) that the resonant angular frequency xcfx89s becomes smaller as the combined inductance Ls of the horizontal correction coils becomes larger, and then the parabolic voltage waveform becomes flatter and the effect of S-correction becomes weaker. When the combined inductance Ls of the horizontal correction coils becomes smaller, the effect of S-correction becomes stronger.
Thus, in the image distortion correcting circuit shown in FIGS. 23 and 24, the combined inductance Ls of the horizontal correction coils becomes larger in the top and bottom areas of the screen and therefore the effect of S-correction becomes weaker to cause outer-area expansion. In the middle area of the screen, the combined inductance Ls of the horizontal correction coils becomes smaller and therefore the effect of S-correction becomes stronger to cause inner-area expansion. The intermediate pincushion distortion can thus be corrected.
As shown in FIG. 24, the conventional image distortion correcting circuit uses three partial cores 13a to 13c to realize the core 13 in the intermediate pincushion distortion correcting saturable reactor unit 10, which leads to increased cost and complicated manufacturing process.
The conventional image distortion correcting circuit has other problems like the following Since the entire system including the core 13 and magnets 11 and 12 does not form a closed magnetic circuit, magnetic leakage occurs at low level. Since the vertical correction coil is rectified by diodes, the vertical deflection sensitivity is deteriorated by the resistance components of the diodes and the consumption power is increased.
A first aspect of the present invention is directed to an image distortion correcting device comprising first and second horizontal correction coils provided on a horizontal deflection current path through which a horizontal deflection current flows, the first and second horizontal correction coils being connected in series and wound around a core in such directions that the first and second horizontal correction coils produce magnetic fields in opposite directions to each other; magnetic field biasing means for biasing the magnetic fields in a first direction; and a vertical correction coil provided on a vertical deflection current path through which a vertical deflection current flows, for producing a magnetic field in a second direction opposite to the first direction, wherein the vertical correction coil is wound over the first and second horizontal correction coils along the periphery of windings thereof.
Preferably, according to a second aspect, in the image distortion correcting device, the core includes first and second partial cores, and the first and second horizontal correction coils include coils wound around the first and second partial cores, respectively.
Preferably, according to a third aspect, in the image distortion correcting device, the core includes an integral single-unit core, and the first and second horizontal correction coils include coils wound in first and second regions of the integral core, respectively.
Preferably, according to a fourth aspect, in the image distortion correcting device, the magnetic field biasing means includes first and second magnets provided at both ends of the core, with their polarities directed in the same direction.
Preferably, according to a fifth aspect, in the image distortion correcting device, the magnetic field biasing means includes a single-unit magnet provided between the first and second partial cores.
Preferably, according to a sixth aspect, the image distortion correcting device further comprises a magnetically closing member connected to at least one of the magnetic field biasing means and the core, for forming a closed magnetic circuit together with the magnetic field biasing means and the core.
Preferably, according to a seventh aspect, in the image distortion correcting device, the magnetically closing member includes a yoke plate arranged to be magnetically coupled with the magnetic field biasing means or the core.
Preferably, according to an eighth aspect, in the image distortion correcting device, the vertical correction coil includes first and second vertical correction coils, the first vertical correction coil produces a magnetic field in the second direction when the vertical deflection current having a first polarity flows, and the second vertical correction coil produces a magnetic field in the second direction when the vertical deflection current having a second polarity flows opposite to the first polarity flows, and the first and second vertical correction coils include coils wound concurrently over the periphery of the first and second horizontal correction coils.
Preferably, according to a ninth aspect, the image distortion correcting device further comprises an insulating bobbin provided along the periphery of windings of the first and second horizontal correction coils, and the vertical correction coil includes a coil wound around the bobbin.
Preferably, according to a tenth aspect, in the image distortion correcting device, the vertical correction coil includes coils wound approximately the same number of turns in first and second peripheral regions respectively corresponding to the first and second horizontal correction coils.
Preferably, according to an eleventh aspect, the image distortion correcting device further comprises an insulating bobbin provided along the periphery of windings of the first and second horizontal correction coils, and the bobbin has a collar in a position corresponding to a middle position between the first and second horizontal correction coils.
Preferably, according to a twelfth aspect, in the image distortion correcting device, at least one of the first and second horizontal correction coils and the vertical correction coil includes a coil using assembled stranded wire as its winding.
As stated above, in the image distortion correcting device of the first aspect of the invention, the vertical correction coil is wound over the first and second horizontal correction coils along the periphery of the windings of them. Therefore no separate core is required for the winding of the vertical correction coil and the cost of the device can thus be reduced.
Furthermore, since the vertical correction coil is wound over the periphery of the first and second horizontal correction coils, wire having a relatively large diameter can be used as the vertical correction coil so as to reduce its resistance component, thus reducing the consumption power of the vertical correction coil.
According to the image distortion correcting device of the second aspect, the first and second horizontal correction coils are wound separately around the first and second partial cores. Accordingly the mutual inductance of the two can be set low relatively easily.
According to the image distortion correcting device of the third aspect, the first and second horizontal correction coils are wound around an integral single-unit core. The use of one integral core for two horizontal correction coils reduces the device cost.
In addition, the first and second horizontal correction coils are free from misalignment of the axes since they are wound around a common integral core.
According to the image distortion correcting device of the fourth aspect, the first and second magnets provided at both ends of the core effectively exert a magnetic field in the first direction between them.
According to the image distortion correcting device of the fifth aspect, a single-unit magnet is provided between the first and second partial cores to produce a magnetic field in the first direction, so that the device cost can be reduced as compared with a structure using a plurality of units of magnets.
Furthermore, in contrast to a device where magnets are arranged at both ends of a core including first and second partial cores, the wire of the first and second horizontal correction coils wound around the first and second partial cores can be easily drawn out, which enhances the efficiency of the device manufacture.
According to the image distortion correcting device of the sixth aspect, the magnetically closing member forms a closed magnetic circuit together with the magnetic field biasing means and the core, so that the magnetic leakage can be effectively prevented.
According to the image distortion correcting device of the seventh aspect, the magnetic force of the magnetic field biasing means or the vertical correction coil can be enhanced by the magnetic coupling of the yoke plate. Therefore the magnetic field biasing means can be reduced in size to reduce the device cost, or the number of turns of the vertical correction coil can be reduced to reduce the consumption power.
According to the image distortion correcting device of the eighth aspect, the first and second vertical correction coils are wound concurrently and the efficiency of the winding work can be thus enhanced. Further, it is possible to reduce variation in resistance component between the first and second vertical correction coils, since the first and second vertical correction coils are wound in almost the same paths.
According to the image distortion correcting device of the ninth aspect, the insulating bobbin present between the first and second horizontal correction coils and the vertical correction coil effectively prevents short-circuit between the first and second horizontal correction coils and the vertical correction coil.
According to the image distortion correcting device of the tenth aspect, the vertical correction coils are wound approximately the same number of turns in the first and second peripheral regions respectively corresponding to the first and second horizontal correction coils. This suppresses occurrence of cross-talk current between the first and second horizontal correction coils and the vertical correction coil, thus reducing the amount of generated heat and increasing the amount of correction to the intermediate pincushion distortion.
According to the image distortion correcting device of the eleventh aspect, the collar formed halfway on the bobbin allows the vertical correction coil to be easily divided into two partial coils wound approximately the same number of turns in the first and second peripheral regions.
According to the image distortion correcting device of the twelfth aspect, at least one of the first and second horizontal correction coils and the vertical correction coil is made of assembled stranded wire as the winding. This suppresses an increase in alternating-current resistance due to skin effect to reduce the amount of generated heat.
The present invention has been made to solve the aforementioned problems, and an object of the present invention is to obtain an image distortion correcting device with reduced cost and lower consumption power.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.